Almost a third of pupils from the independent sector gained at least three As in this summer’s exams compared with just over one-in-10 attending Government-funded schools and colleges.

The results are likely to tighten private school pupils’ grip in places at leading universities, which demand a string of top grades as a basic entry requirement.

It comes after Alan Milburn, the Government’s lead advisor on social mobility, admitted that poor performance at school and college remained the biggest barrier to higher education for thousands of teenagers from poor backgrounds.

In a controversial report published today, he called on institutions to make lower grade offers to “less-advantaged pupils”.

But the comments sparked anger among head teachers’ leaders who warned that it risked lowering standards.

Brian Lightman, general secretary of the Association of School and College Leaders, said: “Saying to disadvantaged students that we have lower expectations about what they can achieve, and their grades for university entry, can send the wrong message.

“The aim should be to encourage all students to succeed, whether that be in an apprenticeship, higher education or other routes.”

<noframes>Interactive chart: How A-level grades vary across private and state schools</noframes>

According to data published by the Department for Education, some 12.5 per cent of pupils gained three A or A*s at A-level this summer, down from 13.1 per cent a year earlier.

The average point score in A-levels – and equivalent vocational qualifications – also dropped in 2012. The average exam was awarded 211.8 points – roughly equivalent to a C – compared with 216.2 a year earlier.

The move follows a toughening up of A-levels in recent years, with pupils being expected to completed fewer modules in more depth and answer longer essay-style questions.

It emerged that pupils from private schools were significantly more likely to score straight As. Figures show that 30.6 per cent hit the target, compared with just 10.7 per cent among pupils from state schools.

Independently-educated schoolchildren also achieved higher average points in each A-level. They scored an average of 242.3 points – close to a B grade – compared with 211.4 in state schools.

In a further disclosure it emerged that boys were also more likely to achieve straight As than girls – 12.7 per cent compared with 12.2 per cent.